Oxygen sensors located in exhaust systems of automobiles monitor the efficiency of the combustion reaction between gasoline and oxygen. When an engine operates at complete efficiency, the ratio of gasoline and oxygen is such that each reactant is completely consumed. Such reactions are deemed to be at “stoichiometry.” When there is less oxygen than the stoichiometric ratio, not all of the gasoline can be consumed in the combustion reaction resulting in increased pollution and unnecessary gasoline consumption. An engine operating under such conditions is said to be running “rich.” On the other hand, when there is less gasoline than required by the stoichiometric ratio, the combustion reaction will produce more nitrogen-oxide pollutants. Under these conditions, an engine is said to be running “lean.” By monitoring the oxygen content of the exhaust produced in the combustion reaction, oxygen sensors send a signal to the automobile's computer indicating whether the engine is running rich or lean. The computer then adjusts gasoline intake to the engine accordingly to achieve stoichiometry. This feedback is intended to maximize fuel efficiency and minimize pollutants.
Oxygen sensors are capable of detecting the oxygen content of exhaust because they are composed of a material that is electrochemically reactive to oxygen. The signal sent to the automobile's computer is a voltage. The degree of richness or leanness relative to stoichiometry is calculated by an air to fuel ratio called lambda. In an ideal sensor, at stoichiometry, lambda will equal one. A rich mixture produces a lambda of less than one and voltages of from about 800 to 900 mV. A lean mixtures produce lambdas greater than one and voltages of from about 80 to about 150 mV.
Unfortunately, certain components of automobile exhaust such as oil detergent additives can damage oxygen sensors. To combat this problem, protective spinel coatings that guard against a variety of poisons and contaminants in the exhaust are used in oxygen sensors. The protective coatings, however, create a further problem—loss of sensor accuracy and specifically “lean shifting.” This means the sensor gives a lean signal even though the mixture is stoichiometric or slightly rich. Differing diffusion rates among the various components in exhaust cause the lean shift. More specifically, the sensor responds to the presence of a small amount of hydrogen that moves faster relative to other exhaust components. Finally, carbon and hydrocarbon contamination of the protective coating can cause a variation in shifts due to the cycle of collection/burning off.
In response to the shifting problems created by the protective coatings the prior art has, in some cases, added a catalyst to the protective coating to react with free hydrogen and hydrocarbons. By facilitating an equilibrium of the various contaminant species, the catalyst prevents the differing diffusion rates that cause shifts, especially lean shifts. While this solution presents advantages over using a protective coating alone, the use of a catalyst presents further drawbacks. First, the catalyst infused protective coating causes slowing of sensor response, especially when exhaust is becoming lean from a rich condition. Second, this solution lacks durability. Catalytic poisons, such as silicon containing compounds, destroy the catalytic properties of the catalyst and thereby destroy its shift correcting capabilities.
It would therefore be desirable to provide a protective coating for sensors that either eliminates or minimizes the disadvantages of the prior art.
In particular, it would be advantageous to provide a protective coating for sensors, which will correct lean and/or rich shifting while simultaneously maintaining sensor response time. It would also be desirable to provide a protective coating for sensors, which will provide durable correction of lean and/or rich shifting without any reduction of sensor response time.